search for: rl195496

Hello, While looking at some internal benchmarks, I found that llvm generates codes with redundant promotion, something like: xor %al, %cl movzbl %cl, %ecx cmp $0x20, %ecx I believe that the promotion stems from the logic in X86TargetLowering::EmitCmp. Comments in the code says, "Do the comparison at i32 if it's smaller, besides the Atom case. This avoids subregister aliasing issues.

...There's a discussion about the underlying x86 micro-arch details here: http://comments.gmane.org/gmane.comp.compilers.llvm.cvs/167221 The conclusion was that we should change how we currently handle these, but we don't want to regress the case that was addressed by: http://reviews.llvm.org/rL195496 There are open bugs with more discussion related to this: https://llvm.org/bugs/show_bug.cgi?id=17113 https://llvm.org/bugs/show_bug.cgi?id=22473 https://llvm.org/bugs/show_bug.cgi?id=22532 https://llvm.org/bugs/show_bug.cgi?id=23155 (cc'ing Kevin in case there's any update on this one)...

...p;r=kOsLCgQzH7N8ptZ7diJD9g&m=4GlWLa5X3i08opHX4qEinKM5A85qGzRUacht2PVZxT8&s=GXM3c7BrHLhnmiVCIrEm9Kpa9mtVMbrW9G8HTTGUc0s&e=> The conclusion was that we should change how we currently handle these, but we don't want to regress the case that was addressed by: http://reviews.llvm.org/rL195496<https://urldefense.proofpoint.com/v2/url?u=http-3A__reviews.llvm.org_rL195496&d=CwMGaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=kOsLCgQzH7N8ptZ7diJD9g&m=4GlWLa5X3i08opHX4qEinKM5A85qGzRUacht2PVZxT8&s=_8T2G8KU33uBBq3_LkXZEnpzN3J8iKCF5Pg8vXHUPEw&e=> There are open bugs with more discu...